Metal halide lamps and high-pressure sodium lamps are high-pressure discharge lamps produced by enclosing a rare gas and a metal halide or sodium in an arc tube, and are superior to the conventional mercury lamps with respect to the color rendition and luminous efficacy. The high-pressure discharge lamps, however, require a considerably high starting voltage as compared to a mercury lamp. Therefore, it is not possible to turn on the high-pressure discharge lamps simply in the same manner as a mercury lamp.
To eliminate the above-mentioned problem, therefore, there has been proposed a high-pressure discharge lamp in which a starting circuit is accommodated in an outer bulb of the discharge lamp (Japanese Patent Laid-Open Nos. 67174/77 and 16475/78). The starting circuit employed in the discharge lamp of this type consists of a bimetallic switch and a filament that are connected in series. Further, the starting circuit is connected in parallel with the arc tube.
Such discharge lamps can be turned on by using a lighting circuit of a conventional mercury lamp, i.e., by using a lighting circuit which is connected to the commercial alternating power supply via a mercury lamp ballast. The starting circuit operates in such a way that, when a power-supply voltage is applied, an electric current flows into a starting circuit consisting of a mercury lamp ballast, a bimetallic switch and a filament, whereby the filament glows. When the bimetallic switch opens in response to the heat of the filament, a pulse of high voltage is generated by the self induction of the mercury lamp ballast and is superposed on the power-supply voltage. The pulse is then applied to the arc tube to initiate the discharge.
At present, there are five types of high-pressure sodium lamps (metal halide lamps) that can be turned on using conventional mercury lamp ballasts, i.e., there are lamps rated at 220 (250) watts, 270 (300) watts, 360 (400) watts, 660 (700) watts, and 940 (1000) watts. These lamps are turned on by using mercury lamp ballasts having output capacities of 250 watts, 300 watts, 400 watts, 700 watts and 1000 watts, respectively. From the standpoint of saving energy and the tendency today to use high-pressure discharge lamps indoors, however, it has been strongly urged to develop metal halide lamps and high-pressure sodium lamps which consume less energy, i.e., to develop metal halide lamps and high-pressure sodium lamps having small outputs.
To cope with such requirements, an arc tube which can be turned on using a mercury lamp ballast having a capacity of smaller than 200 watts was desired and produced, and the aforementioned starting circuit was accommodated in the bulb to produce a metal halide lamp as well as a high-pressure sodium lamp. These lamps were tested with regard to their starting characteristics, and certain problems were found. Namely, when the power-supply voltage is applied, the starting circuit works to turn on the lamp once. The lamp, however, develops an extinguishing voltage phenomenon immediately after it is turned on so that the properly turned-on state is not sustained. This is a serious problem which hinders the realization of metal halide lamps or high-pressure sodium lamps having small outputs.